1. Field of the Invention
The present invention relates to an information processing apparatus, such as a portable radio card reader/writer, for performing radio communication with a radio card functioning as a portable information storage medium with a radio communication function, thereby performing predetermined information processing such as data read/write.
2. Description of the Related Art
Recently, a so-called radio card has been developed and put to practical use as a portable information storage medium with a radio communication function. In general, in a radio card system using this type of radio card, a radio card reader/writer is used to perform radio data transmission/reception with the radio card, thereby performing predetermined information processing such as data read/write with the radio card.
The radio card reader/writer has an antenna for radio data transmission/reception with the radio card. The antenna may be a single antenna system or a double antenna system. The single antenna system is used to perform semi-dual communication, and in this case a single antenna is used as a transmission antenna and a reception antenna by switching.
If full dual communication is performed by the single antenna system, the structure of signal processing circuits becomes complex. Thus, in general, the double antenna system is adopted, and one of two antennas is used as a transmission antenna and the other as a reception antenna. With this structure, the reception is used only for receiving return signals from the radio card.
According to the conventional double antenna system, however, there is a problem of interference in which a signal from the transmission antenna enters the reception antenna. In particular, in the case of a portable radio card reader/writer, a transmission antenna and a reception antenna are generally arranged close to each other. Thus, the interference is conspicuous.
In general, a weak reception signal from the radio card is processed on the reception side, and it is therefore necessary to increase the degree of amplification. As a result, a signal emitted from the radio card reader/writer is easily received by the reception side of the radio card reader/writer, and noise mixing in the signal processing further increases.
In the prior art, in order to solve this problem, a plural stages of filters are provided in a reception circuit, thereby to eliminate an unnecessary reception signal. In this method, however, the number of filters increases and the structure of the system becomes complex. Thus, the manufacturing cost increases.